The present invention relates to combustion products detectors, and particularly to such detectors of the type which periodically sample the ambient air for the presence of combustion products. The invention has particular application to combustion products detectors of the photoelectric type.
Combustion products detectors for detecting smoke or other particulate airborne combustion products are generally of two types, viz., the ionization type and the photoelectric type. In the photoelectric-type detector, a light source illuminates a darkened chamber into which ambient air is admitted. Combustion products scatter the light to a photoelectric sensor which produces a signal indicative of the presence of the combustion products. Commonly, such detectors actuate the light source periodically, the sampling period preferably being rather long so as to minimize power consumption.
Some such detectors are designed for operation from an AC power source. But there is a large amount of electrical noise present on any commercial AC power line which tends to disrupt the normal operation of the smoke detecting circuits. Thus, it is necessary to eliminate most of this noise. Since this noise is at a maximum during the peaks of the AC line voltage and is at a minimum at the line zero crossings, it is known to so arrange the sampling circuit that the sampling occurs only at or very near the zero crossings of the AC line voltage. Such circuits have, heretofore, been resistively coupled to the AC line.
It is desirable to capacitively couple the sampling circuitry to the AC supply to further minimize power consumption. Such capacitive coupling can reduce power consumption by causing the supply current drawn from the AC line to be almost 90.degree. out of phase with the AC line voltage, thereby significantly improving the power factor. However, prior periodic sampling circuits which sample at the zero crossings are incompatible with capacitive coupling, because the phase difference between the power line voltage and current adversely affects the operation of the zero crossing circuitry.